Prime movers for driving a generator are subjected to the legal requirement that within a stipulated short period of time after interruption of services a predetermined electric voltage must be supplied.
Therefore, the prime movers for a generator must satisfy two requirements, one being that time required for start-up of prime mover be short enough to meet the above-mentioned legal requirement and, the other being that variation in rotational speed of a power turbine shaft of prime mover against load variations imposed on the power turbine shaft be small with a view for maintaining its output voltage frequencies constant.
It is well known that there are two types of gas turbines, i.e. a single-shaft type and a twin-shaft type.
Twin-shaft gas turbine engines are suitable as a prime mover for driving a generator in that start-up time is short enough to satisfy the first requirement because a compressor drive shaft is accelerated irrespectively of a power turbine shaft. However, since, in the twin-shaft gas turbine engines, inertia moment of a power turbine shaft is not great, there is a considerably large variation in rotational speed of the power turbine shaft against load variations imposed on the power turbine, whereby they fail to satisfy the second requirement.
In a known system, the rotational speed of the power turbine shaft is controlled by controlling the amount of fuel supplied to the combuster of the gas turbine. The fuel is controlled in accordance with a signal produced by detecting the difference between the rotational speed of the power turbine shaft and a reference signal representative of a rated or desired rotational speed. This kind of closed loop fuel control system works effectively when the variation of the rotational speed of the power turbine shaft occurs gradually or slowly. However, when the rotational speed of the power turbine shaft varies suddenly for instance when the load is connected or disconnected, abovementioned conventional type of closed loop fuel control system does not perform adequate feedback control and thus the rotational speed is apt to move out of an allowable range.